A forecast slump in commodities exports to China has sparked the first downward revision to company tax receipts since the COVID-19 pandemic and will worsen the budget bottom line by $8.5 billion over the next four years.

Treasurer Jim Chalmers said the mid-year budget update released on Wednesday would show a deterioration in the forecast deficit of $28.3 billion this financial year, which he blamed on rising government spending and falling company tax revenue.